Hey Reliys!,
Don't have enough time to give a furrow guide, but I believe this should be enough to get you in the right direction of doing what you want. To begin with, making custom textures is actually a lot easier than you'd probably think, I remember when making a custom texture for some of my maps, I used BLP Converter which basically converts Wows texture format (In wotlk, BLP) into PNG and back, when you have your texture, you could simply replace an existing one, or if you wanted a new texture entirely, you would make a folder in your MPQ patch with something like:
Patch/TILESET/Texturesetname/Texture.blp
--> Here is a link to BLP converter, although it should be in the WowDevKit in the downloads section of the website: http://www.modcraft.io/index.php?topic=801.0
Then, making the map, I'd recommend simply following this tutorial: https://model-changing.net/tutorials/article/22-24-creating-your-own-custom-blank-map/
If you need any more help, If I have some time, you can send me your watermark and I may be able to do it for you, also, if I did that, I wouldn't want any money cause I don't do wow modding for any monetary value, just like helping others out. : )